Identification: Norway rats typically have coarse, brown
fur with a pale gray or grayish brown underside. They have small eyes, naked
ears, and a scaly tail that is shorter than the length of their head and body.
Mature rats are range between 150-300 grams and about 400 mm long. The females
have 12 mammae.

Original Distribution: Rattus norvegicus are native
to central Asia and believed to have come from Northern or Northeastern China.
They did not come from Norway, as their name suggests, but they are believed
to have entered Europe in the mid 1500s on ships from Norway.

Current distribution:
They are generally considered to have a worldwide distribution, where
they frequent temperate regions and most recently invaded many islands. Like
some other rodent species, they are commonly found in all major cities of
the world because they are associated with humans.

Site and Date of Introduction: Norway rats are believed
to have first arrived in the United States on boats around 1776. They were
brought over in boxes of grain by Hessian troops who were hired by Britain
to fight the American colonists.

Mode(s) of Introduction:
Norway rats have primarily been introduced to new areas by being transported
on ships in cargo or by stowing away. With increases in trade and new methods
of travel, it is likely that they have also been transported on trains and
automobiles. They are also excellent swimmers and may have entered new regions
through self-propelled swimming.In 1727, large masses of these rats
swam across the Volga River and entered into Russia. Migrating rats can swim
up to half a mile and survive by treading water for 3 days.

Reason(s) Why it has Become Established: The primary reasons
Rattus norvegicus has become established are its high reproductive
rates and its association with humans. Norway rats have evolved and adapted
to urban areas as a result of their opportunistic and omnivorous behavior.
Due to this rat’s large size and aggression, they are strong competitors and
tend to displace other rodent species. They can survive in a variety of different
habitats and take advantage of shelters and resources provided by vacant
buildings, subways, crop fields, and salt marshes. They have also adapted
to different climates, which is enabled by human habitation and a constant
supply of food. Rat pups have developed a behavior called "huddling" that
not only helps them keep warm, but also contributes to the cohesion of their
group. In addition, Norway rats are successful edge species preying on bird
eggs and other food which allows them to flourish in fragmented habitats.
Within cities, Norway rats are found in the cellars and basements of houses
and they thrive in sewers. Though they live in large, closely knit groups,
they are conspicuous since they can hide in small places. This species is
not entirely released from their natural predators like snakes, owls, hawks,
weasels, and minks, but rather domestic cats and dogs prey on rats in cities.
However, they have no significant effect on decreasing rat populations. Mortality
rates for city rats are high, but they are compensated by high fecundity and
the availability hiding places. They build underground burrows with long tunnels
and chambers for food storage and nests. Their nests are found either inside
or outside, usually located in sewers, storm drains, crawl spaces, storage
rooms, and in any cluttered area. To find resources, they will travel up
to 2-3 miles, though their normal home range is about 50 meters. Their only
limitations are food and water, which they need between 0.5-1 ounce per day
to survive. Rats can survive for 14 days without food and will resort to
cannibalism if none is available. They will also carry out massive migrations
to new areas when crowding occurs, increasing their rate of spread. Once they
are introduced to a new area, their populations increase rapidly as they
take advantage of nearby resources to reproduce and become established.

Ecological
Role: Norway rats are nocturnal and omnivorous species who are ground
dwellers living mostly in cities and crop fields. They are active all year
round and can eat about third of their weight in one day. Food for these
rats includes anything from garbage, soap, candy, fruits, grains, seeds and
even other rodents and animals. Rattus norvegicus live in large groups
ranging from 15 to 220 individuals where there is usually a dominance hierarchy.
Their social structure is characterized by a dominant male, who defends his
territory and prevents other males from mating, and a harem of females who
defend their resources and nests. Norway rats are prolific breeders and will
reproduce throughout the year, which is mostly determined by food availability.
Females usually have about 7 young per litter and 6-8 litters per year. They
also experience a postpartum estrus and are capable of mating 18 hours after
giving birth. Young are completely independent after about four weeks and
they reach sexual maturity at 3 months. However, males tend to reproduce later
when they are larger and can challenge dominant males. Norway rats will reproduce
until they are 1.5-2 years old, where the life span is about 3 years old.
As a result, their opportunistic behavior and high reproductive rates allow
them recover rapidly from disease and control attempts and continue increasing
the spread and size of their populations.

Benefit(s):
This species has been selectively bred to create the albino lab rat
used for research purposes. Some forms of this species are also kept as pets.
To a smaller degree, they serve as decomposers in cities by reducing garbage
and other food lying around.

Threat(s):
Some people consider Rattus norvegicus to be the greatest mammal
pest of all time. Currently, an estimated 150-175 million Norway rats live
in the United States causing millions in dollars in damage to crops and buildings
each year. A U.S. Government report estimated that each individual rat annually
damages $1 to $10 worth of food and other material, and contaminates 5 to
10 times more. Using their ever-growing incisors and strong jaws, these rats
cause structural damage by burrowing underneath buildings and walkways and
gnawing through walls, pipes, and electrical wires. These rats have even started
fires by gnawing matches and have caused floods by tunneling through dams.
They contaminate crops and food, and may also restrict plant growth by eating
large amounts of seeds. More importantly, they transmit diseases directly
by biting people and contaminating food, and indirectly by carrying lice
and fleas. Historically, they have been vectors for bubonic plague, leptospirosis,
typhus, spotted fever, tularemia, salmonella food poisoning, infectious jaundice,
and other serious diseases.

Control
Level Diagnosis: It has been recommended that Rattus norvegicus
receive high priority diagnosis for control because they are a highly invasive
species causing large amounts of damage each year. They have contributed
to the extinction and reduction of many mammal, bird, reptile, and invertebrate
species through competition and predation. They also pose a threat to human
health as vectors of many diseases. Since they are commensal with people,
as human population increase, so do the rat populations. Today, there is
practically a one to one ratio of people to Norway rats in cities. Thus,
Norway rat populations should be closely monitored and eradicated when necessary.

Control Method: A
variety of control methods have been used including snap traps, habitat
destruction, and chemical control. Some rats have learned to avoid traps,
while others have developed resistance to poisons. Another method has involved
introducing natural predators like the barn owl to reduce populations. Though
there are significant risks in introducing another species into new areas,
in Malaysia, introduced barn owls were successful in killing about 1,300
rats a year. The most effective control methods usually include a combination
of many techniques. In some areas, it is recommended that people “rat proof”
their homes by removing wood piles, keeping their yards cut low, making sure
food is stored in safe containers, and filling all interior and exterior
holes in buildings to prevent rats in the walls. This includes:

1. Blocking all
openings around pipes, utility lines, and air vents2. Sealing doors and
windows3. Sealing holes and
cracks in the foundation4. Repairing and sealing
all openings on the roof5. Repairing gnaw holes

Rodenticides have been the most common control method in reducing rat populations.
However, there are many risks in using these poisons indoors. For example,
if a rat ingests some poison and crawls back in the walls to die, this could
result in foul odors and the arrival of beetles or other insects that will
feed on the dead rat. On the other hand, eradication has been successful on
some islands using poisons. Despite this, there has been some evidence that
they can survive and even adapt to poison. A years ago, Warfarin was developed
as a rat poison, but within the year many rats were seen eating it for food.
Norway rats can also learn to avoid poison. Even though their eyesight is
poor, they have an excellent sense of smell and can detect contaminants in
their food at only 0.5 parts per million. Also, the rat's sense of taste is
so strong it can detect as little as two parts per million of poison in their
food. Thus, to control rat populations, careful monitoring and the use of
many different control methods must be used.